1. Field of the Invention
The present invention relates to a misfire detecting apparatus for an internal combustion engine and, particularly, to a misfire detecting apparatus for determining whether a misfire is present based on a rotational speed parameter according to an engine rotational speed.
2. Description of the Related Art
Japanese Patent Laid-open No. H9-119338 discloses a conventional method for determining whether a misfire is present on a featuring signal q(n) that is based on a segment time period, which is a period of time that is required for a rotation of a predetermined crank angle. The featuring signal q(n) is obtained by converting a signal of a segment time period represented as a point on a complex plane, wherein the misfire determination is performed based on an amplitude and a phase of the featuring signal q(n).
In the above-described conventional method, a band pass filter is required for extracting a desired frequency component from the signal indicating the segment time period as a rotational speed parameter. Further, it is necessary to change the filtering characteristic according to a misfire pattern (occurrence mode of misfire, for example, continuing misfire in one cylinder, continuing misfire in two cylinders facing each other, and the like) to be detected. Therefore, the misfire determination process of the conventional method is rather complicated.
Further, in the conventional method, the cylinder in which misfire is occurring may not be accurately identifiable, details of the misfire determination process must be changed depending on the number of cylinders of the engine, and the manpower necessary for setting a determination threshold value is relatively large.